The present invention relates to covers for underground structures, and more particularly to covers for underground structures which are designed to more efficiently prevent tires of vehicles such as motorcycles from slipping and skidding.
Covers for underground structures refers herein to large iron covers to close openings which connect buried materials as well as structural sewerage facilities to above ground, manhole covers, covers of house inlets, openable and closable iron covers for common-use tunnels which protect apparatus and equipment for underground power and communications facilities, iron covers for electric power transmission, iron covers for power distribution, fire hydrant covers functioning as openable and closable doors to connect underground conduits in waterworks systems, gas pipes and their accessories to above the ground, sluice valve covers, air-valve covers, covers for gas distributing pipes, and water-gauge valves.
On the surfaces of the currently available bodies of covers for underground structures placed on sidewalks and roadways are formed protruding and recessed patterns which serve as decorations as well to prevent pedestrians from slipping and vehicles from skidding in inclement weather.
Examples of the cover bodies with improved surface patterns for slip and skid prevention are available in Japanese Unexamined Utility Model Publication No. 42252/83 and Japanese Unexamined Utility Model Publication No. 86156/88.
The models disclosed therein are concerned with a cover body on the surface of which is formed projections on top of which are formed small protrusions and recesses, or small protrusions being formed on recessed portions in areas other than the projections. In other words, these models seek to expand the surface coarseness of consecutive protrusions or depressions by adding small protrusions and recesses to the protruding and recessed patterns on top of the cover body and to improve anti-slip and anti-skid performance by increasing frictional resistance.
Nonetheless, the small protrusions and recesses provided on top of the projections on the surface of the cover body come into contact with tires whenever vehicles pass and rub therewith, thereby wearing out with the passage of time after installation, with a resultant reduction of frictional resistance leading to diminished anti-slip and anti-skid effect.
Even immediately after the installation, as dirt, rainwater, and other materials accumulate on the recessed portions of small projections and recesses, the frictional resistance will not effectively act upon the tires, and anti-slip and anti-skid effect will likewise deteriorate.
In this manner, the conventional structure purports to enlarge the surface coarseness of the cover body and to increase frictional resistance, whereas the more provision of small protrusions and recesses fails to maintain the anti-slip and anti-skid performance over a long period of time. Another problem is that when dirt, rainwater, and other materials are involved, it is impossible to bring the anti-slip and anti-skid performance into play at the outset.
These problems are to be resolved by the cover for underground structures according to this invention which can maintain excellent anti-slip and anti-skid performance over a long period of time regardless of the installation environment, through optimization of arrangement patterns of projections provided on the surface of the cover body as well as the shape of the projections.